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Acta Univ. Palacki. Olomuc., Gymn. 2011, vol. 41, no. 2
ISOKINETIC STRENGTH OF KNEE FLEXORS AND EXTENSORS OF ADOLESCENT
SOCCER PLAYERS AND ITS CHANGES BASED ON MOVEMENT SPEED AND AGE
Michal Lehnert, Josef Urban, Jiří Herbert Procházka, Rudolf Psotta
Faculty of Physical Culture, Palacký University, Olomouc, Czech Republic
Submitted in August, 2011
BACKGROUND: During childhood and adolescence there is growth in muscle strength. The dynamics of change
in muscle strength and its causes have not yet been fully clarified and may differ within individuals and specific groups.
The knowledge of current isokinetic strength levels among athletes of varying ages and disciplines represents important
information from the point of view of sport performance, as well as health prevention.
OBJECTIVE: The aim of this study is to determine within a group of highly trained adolescent soccer players
(n = 45; age = 17 ± 1.2 years; body height 178.4 ± 5.3 cm, body weight 68.5 ± 7.6 kg) the isokinetic strength of knee
flexors and extensors and to judge whether it is significantly influenced by age and movement speed.
METHODS: A group of players was further divided according to age into 3 subgroups – U16 (n = 16); U17 (n = 14);
U18 (n = 15). Unilateral strength was measured by the isokinetic dynamometer ISOMED 2000 (D. & R. Ferstl GmbH,
Hemau, Germany) at angular speeds of 60° × s–1, 180° × s–1 and 360° × s–1. The parameter evaluated was the isokinetic
peak torque (PT; Nm).
RESULTS: The ANOVA results demonstrate that there was a significant decrease in the PT within the entire group
of players with an increase in speed during both flexion and extension (dominant extremity: p < 0.001 resp. p < 0.019;
non-dominant extremity: p < 0.001; resp. p < 0.001). The difference in PT among the age categories was not significant for both flexors and extensors at the speed of 60° × s–1 (p = 0.005; resp. p = 0.036) and the speed of 180° × s–1
(p = 0.036; resp. p = 0.033). However, significant differences in PT among individual categories were confirmed only
in some cases and, by contrast, an insignificant decrease was marked with nondominant leg extensors.
CONCLUSIONS: The results of this study indicated the state of the strength readiness of the highly trained soccer
players in the junior category and the potential shortcomings in the concept of strength preparation within a leading
club in the Czech Republic. These results contributed to the deepening of knowledge about changes in strength with
age and confirmed the importance of applying diagnostics during systematic long-term sport preparation.
Keywords: Soccer, isokinetics, diagnostics, youth.
INTRODUCTION
It is generally accepted that during childhood and
adolescence muscle strength changes take place. This
is, however, a complicated problem, since these changes
can progress differently in terms of inter-individual differences (maturation, body size, etc.), muscle groups,
types of muscle action, sex specificity, etc. Body height
and the lever armʼs role in muscle strength play a key
role in strength development; however, some changes
during development are independent of body size and
are probably caused by the development of the nervous
system. Understanding this field is also influenced by
nuances in the applied testing procedures (Bar-Or, 1996;
De Ste Croix, 2007; Faigenbaum, 2008; Forbes, Bullers
et al., 2009).
Isokinetic dynamometry is considered an objective
and reliable diagnostic tool. Its foremost advantages are
high reliability, standardization, the possibility to identify
the strength level in the entire extent of the movement
and determine in this way positions, at which the best
and the worst value of PT is reached (Baltzopoulos &
Brodie, 1989; Dvir, 2004). Isokinetic strength measurements are numerous in the literature; however, fewer pieces of work have been focused on the evaluated changes
of the isokinetic strength of the lower extremities in children and youth (Bellew & Malone, 2000; Weir, 2000).
The questions of developmental changes in isokinetic
strength were summarized in the overview study by De
Ste Croix, Deighan and Armstrong (2003), who, among
other things, note that although some information on the
strength relationships between the knee extensors and
flexors for children exist, data on the age trends which
affect these muscles are limited. The results of these isokinetic measurements also confirm that the PT values are
influenced by the speeds used during testing and that, in
accordance with the Hill Curve, the PT decreases with an
increasing speed of movement (Chan & Maffulli, 1996).
46
Isokinetic testing of muscle strength is used for evaluating the effects of training programs, especially for
identifying muscle weakening and its compensation
and for the identification of imbalance and prior injuries (Baltzopoulos & Brodie, 1989; Brown, 2000; Houweling, Head, & Hamzeh, 2009; Perrine, 1993). The
number of injuries to young players is lower than the
number of mature highly trained soccer players, but the
treatment time is longer for young players (Hawkins et
al., 2001). The number of injuries increases according
to increasing age and in the age category from 17 to
20 years injury occurrence is almost the same as in the
adult category (Bahr et al., 2008).
The results of case studies concentrating on the
strength of young soccer players’ lower limbs have confirmed the rising trend in strength related to chronolo gical age. Results are not definite regarding the
developmental period, flexors or respectively, extensors,
the contraction type and speed measurement. The study
of young soccer players aged from 12 to 18 years (Forbes,
Sutcliffe et al., 2009) has confirmed that the absolute
PT increases with the chronological age of the players.
The rise of PT was almost linear in the age group from
11 to 14 years, then the major PT growth was in the age
group from 15 to 18 years and then, in the age group
from 18 to 20 years, the growth was almost linear again.
Authors point out that a similar trend has not been
proven in tennis players nor in youth without a sport
specialization. That is the reason why the authors suppose that soccer training specifically has a strong impact on muscle strength development and particularly
on lower limb progress. Kellis S., Gerodimos, Kellis E.
and Manou (2001) note that age affects the strength of
the dominant leg (DL) and non-dominant leg (NL) and
that in the age growth period young soccer players gain
better trained lower limbs and the bilateral difference
decreases. Gür, Akova, Pündük and Küçükoğlu (1999)
monitored a group of players younger than 21 years old
and a group of players older than 21 and they found that
PT grows with age only in the dominant leg. Authors
supposed that the differences were present mostly due to
training elements rather than age factors. Forbes, Bullers
et al. (2009) pointed out that the influence of the actual pubertal state compared to the maximal amount of
strength is higher than that of age itself. Lehance, Binet,
Bury and Croisier (2009) compared the flexors’ and extensors’ knee strength of the player groups U17, U21 and
adult players at the speed of 60° × s–1 and 240° × s–1. They
found absolute growth of the concentric PT in all cases.
Statistically significant differences for both speeds were
identified only for the concentric extensors’ strength.
No significant difference was confirmed by expressing
PT with regard to body weight. Higher growth in extensors compared to the flexors could be, according to
the authors, caused by a lack of attention to hamstring
Acta Univ. Palacki. Olomuc., Gymn. 2011, vol. 41, no. 2
strengthening. The results of some studies do not correspond with the trend of increasing strength related to
chronological age. For example, De Ste Croix, Deighan
and Armstrong (2003) have pointed out that, although
age has a strong effect on strength development, the rate
of anatomical growth and maturation vary independently
and their effects do not correlate to chronological age.
It is clear that the questions about isokinetic knee
flexors’ and the extensors’ strength of the DL and NL
within the different age periods, in different specific
groups and according to the speed measured are not
yet well answered. The goal of this study is to determine
the isokinetic knee strength of flexors and extensors by
highly trained adolescent soccer players and to evaluate whether the chronological age and the speed of the
movement significantly affect the strength of the knee
joint flexors and extensors.
METHODOLOGY
The group of tested soccer players was formed
by 45 members of the SK Sigma soccer team (age
17 ± 1.2 years; height 178.4 ± 5.3 cm; weight 68.5 ± 7.6 kg).
44 players had DL on their right, 1 player had DL on
their left (a dominant lower limb was determined as the
one which is preferred by players for kicking the ball).
The players were divided into three subgroups according
to their age category U16 (n = 16; height 175.5 ± 4.9 cm;
weight 63.4 ± 6.9), U17 (n = 14; height 181.9 ± 4.1 cm;
weight 73.4 ± 7.2), U18 (n = 15; height 181.0 ± 5.0 cm;
weight 69.2 ± 5.7). All tested players, as well as their
parents, were fully informed about the goal and methodology of the measurements. They agreed with the testing
process and with the use of the data for further research.
Players with acute medical problems were excluded from
the research. The day before testing, the players were not
exposed to any high training pressure.
Just before testing the players went through nonspecific warm up exercises, which included cycling on
a bicycle ergometer for 6 minutes with a rising load
and 5 minutes of auto-stretching exercises controlled
by a physiotherapist. Unilateral strength of the concentric contraction of the flexors and extensors of the
knee was measured with an isokinetic dynamometer
ISOMED 2000 (D. & R. Ferstl GmbH, Hemau, Germany). Players were tested in the sitting position and
they held handgrips alongside the seat, the seat back
was reclined by 15 grades. For fixation in the area of the
pelvis and thigh fixed belts were used, shoulders were
fixed by a shoulder rest in the ventral-dorsal and cranialcaudal direction. The axis of the dynamometer rotation
was the same as the axis of the knee rotation (lateral
femoral condyle). The arm of the dynamometer lever
was fixed to the distal part of the lower leg and the lower
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Acta Univ. Palacki. Olomuc., Gymn. 2011, vol. 41, no. 2
edge of the training pad was placed 2.5 cm over the medial apex malleolus. The seat settings were stored in PC
memory before measuring the right leg and were automatically activated in the process of measuring the left
leg. The angular speed parameters 60° × s–1, 180° × s–1
and 360° × s–1 were used for measurement and the gravitational correction was activated as well. The testing
protocol was made up of two contraction sets in each
of the measured speed values. In the first – the warm
up set – players made five reciprocal contractions (concentric contraction to flexion, followed by concentric
contraction to the extension) with a progressive raise in
the contraction strength of the player until the maximal
contraction strength was achieved. After a 30 seconds
rest period, a set of six repetitions with the maximal
contraction strength followed. The rest time during the
measurements at particular speeds was 1 minute and
the rest time between the measurement of the right and
left leg was 3 minutes. During the testing process, the
players were motivated to reach the best results and they
were provided with concurrent visual feed back in the
form of an isokinetic strength curve displayed on the
dynamometer monitor. The result of the measurement
was the absolute PT (Nm) in the process of concentric
flexion, respectively extension in the knee joint.
The two way ANOVA was used to determine the significance of the PT differences between groups (p < 0.05).
The statistical significance of the differences between
age categories was verified by the post hoc – LSD test.
Effect size was assessed by the coefficient “Eta square”
(0.01–0.05 low effect; 0.06–0.13 middle effect; > 0.14
large effect); (Cohen, 1988). Statistical analysis was per-
formed using the data analysis software system STATISTICA, version 10.
RESULTS
Isokinetic muscle strength DL and NL values at
various speeds by the entire tested group of soccer
players for given age categories are noted in TABLE 1
and TABLE 2. As expected, the tested players reached
higher values for extensors than flexors at all speeds.
The highest values were measured at a speed of 60° × s–1
for all players and for all age categories. Evaluation of
isokinetic muscle strength at three angular speeds for
the entire group confirmed that PT was significantly
lower for DL and NL with an increasing speed for
both flexion and extension. The value of the effect
for these differences was high [DL: F(2.84) = 99.80;
p < 0.001; η2 = 0.703, resp. F(2.84) = 327.35; p < 0.001;
η2 = 0.886]; [NL: F(2.84) = 91.26; p < 0.001; η2 = 0.684;
resp. F(2.84) = 267.27; p < 0.001; η2 = 0.864]. Detailed
comparison of the average values reached at particular speeds with the help of the LSD post – hoc test
confirmed statistically significant differences between
all speeds used (not mentioned in the enclosed tables).
ANOVA results (TABLES 3–5) show statistically
significant differences in muscle strength (without consideration for lateral preferences) among age categories
for flexors as well as extensors at the speed of 60° × s–1
and 180° × s–1. The size of these differences was high.
At the speed of 360° × s–1 important differences were
not confirmed.
TABLE 1
Peak torque (Nm) by the flexion of the knee joint for the dominant and non-dominant leg – basic statistical characteristics (n = 45)
Flexion DL
M
U17
Med
SD
123.3
121,0
19,37
120.6
120.0
18.61
102.6
102.0
18.17
102.0
106.0
17.31
–1
79.2
82.0
20.79
80.8
85.0
23.62
60° × s–1
113.0
115.0
18.21
109.9
112.5
17.29
180° × s
–1
93.6
94.5
16.19
94.8
96.0
17.00
360°× s
–1
73.9
78.0
13.40
72.5
69.5
20.04
60° × s–1
125.6
122.0
15.33
123.2
124.0
13.27
–1
180° × s
106.3
107.5
18.29
103.0
106.0
18.97
360° × s–1
78.8
84.5
27.46
84.0
88.0
27.99
60° × s
U18
M
60° × s
360° × s
U16
SD
180° × s–1
–1
Entire group
Flexion NL
Med
–1
132.0
130.0
19.96
129.6
136.0
19.53
180° × s–1
108.8
105.0
17.27
108.9
111.0
13.70
360° × s
85.0
87.0
19.94
86.6
90.0
21.68
–1
Legend: M – average, Med – median, SD – standard deviation, DL – dominant leg, NL – non-dominant leg
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Acta Univ. Palacki. Olomuc., Gymn. 2011, vol. 41, no. 2
TABLE 2
Peak torque (Nm) by the extension of the knee joint for the dominant and non-dominant leg – basic statistics characteristics (n = 45)
Extension DL
60° × s
Entire group
U16
U17
–1
180° × s–1
M
Med
SD
M
Med
SD
212.9
213.0
35.93
209.7
208.0
33.32
152.5
151.0
21.99
150.7
144.0
22.26
360° × s–1
115.4
118.0
26.54
114.8
115.0
29.73
60° × s–1
189.2
189.5
34.00
192.3
209.6
32.32
180° × s–1
139.9
139.5
17.41
142.5
140.0
16.02
360° × s–1
106.3
106.0
21.88
106.7
108.0
22.80
60° × s–1
226.3
227.0
23.00
222.5
222.5
25.66
180° × s–1
159.9
165.0
17.44
156.2
159.5
25.79
360° × s
U18
Extension NL
–1
121.8
128.0
29.23
123.1
132.0
39.29
60° × s–1
225.8
216.0
36.42
216.2
217.0
34.72
180° × s
159.0
151.0
25.13
154.4
148.0
23.27
119.1
127.0
27.49
115.8
120.0
25.30
–1
360° × s
–1
Legend: M – average, Med – median, SD – standard deviation, DL – dominant leg, NL – non-dominant leg
TABLE 3
Differences in the isokinetic strength by flexion and extension of the knee joint among the age categories at the speed
60°× s–1 (n = 45)
FLEXION
EXTENSION
F
p
η2
0.217
6.103
0.036
0.225
0.199
0.038
0.971
0.330
0.022
0.987
0.000
1.164
0.321
0.052
F
p
η
Age
5.821
0.005
Lat
1.703
Age × Lat
0.013
2
Legend: Lat – laterality factor, F – testing criteria level, p – level of statistical significance, η2 – Eta square
Statistically significant values are in bold characters
TABLE 4
Differences in the isokinetic strength by flexion and extension of the knee joint among the age categories at the speed
180° × s–1 (n = 45)
FLEXION
F
p
EXTENSION
η2
F
p
η2
Age
3.595
0.036
0.146
3.675
0.033
0.148
Lat
0.134
0.716
0.003
0.537
0.467
0.012
Age × Lat
0.480
0.622
0.022
0.771
0.469
0.035
Legend: Lat– laterality factor, F – testing criteria level, p – level of statistical significance, η – Eta square
Statistically significant values are in bold characters
2
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Acta Univ. Palacki. Olomuc., Gymn. 2011, vol. 41, no. 2
TABLE 5
Differences in the isokinetic strength by flexion and extension of the knee joint among the age categories for the speed
360° × s–1 (n = 45)
FLEXION
EXTENSION
F
p
η2
1.466
0.242
0.065
0.036
0.849
0.000
0.784
0.011
F
p
η
Age
1.543
0.225
0.068
Lat
0.480
0.492
0.011
Age × Lat
0.550
0.580
0.025
0.243
2
Legend: Lat – laterality factor, F – testing criteria level, p – level of statistical significance, η – Eta square
Statistically significant values are in bold characters
2
The LSD post – hoc test, on the other hand, confirmed
significant differences only in some cases between categories. For the speed of 60° × s–1 by DL between the categories of U16 and U 18, in flexion (p = 0.005), between the
categories of U16 and U17, U16 and U18 in extension
(p < 0.001, resp. p < 0.001). Between categories U17 and
U18 no significant differences were confirmed. For NL
a significant difference was confirmed for the categories
U16 and U18, in the case of flexion and also extension
(p = 0.005, resp. p = 0.019) and between U16 and U17
in extension (p = 0.004). Differences between the categories U17 and U18 were not statistically significant. At
the speed of 180 × s–11 for DL, statistically significant
differences were confirmed for the categories of U16 and
U17 in extension (p = 0.045) and U16 and U18 in flexion
and extension (p = 0.025, resp. p = 0.048). In NL, statistically significant differences were confirmed only for the
U16 and U18 categories (p = 0.044) in flexion.
DISCUSSION
The knowledge of the muscle strength of the knee
joint flexors and extensors in soccer players is necessary from the point of view of productivity as well as injury prevention (Lehance et al., 2009). The average
PT for the players from our group was at the speed of
60° × s–1 for DL for flexion 123.3 Nm, for extension
212.9 Nm, for NL 120.6 Nm, resp. 209.7 Nm. When we
compare the average PT values of our group of players
with those values noted by Lehance et al. (2009) during flexion, resp. DL extension, with Belgian players
in the category U17 (128 Nm, resp. 194 Nm) we can
say that both groups of players reached similar results.
In comparison with values measured by Forbes et al.
(2009) with 18 year-old British soccer players at the
speed of 60° × s–1, the players from our group were substantially stronger (DL 99.7 Nm, resp. 181.8 Nm, NL
94,5 Nm, resp. 182.7 Nm). We can also compare results
from our tested group with the results of the group from
the same club, but in the category U19 (Botek et al.,
2010). The older players were substantially stronger (DL
165.36 Nm versus 233.64 Nm; NL 169.86 Nm versus
243.93 Nm) and even if we consider the higher average
weight of the older group (M = 73.4 ± 4.9) we assume
that these differences could not have been caused only
by age, but also by the goals and content of the training
process.
A further finding of our study was that PT values
in flexion and extension for DL and also NL were significantly lower with increasing speed. This trend was
also confirmed by Dvir and David (1996); Lehance et
al. (2009); Malý, Zahálka and Malá (2010); Kellis et
al. (2001). These authors explain this decrease by the
fact, that, at high speed levels, muscle contraction is accomplished only by a fraction of the muscle fiber, mostly
that muscle fiber, which ismorphologically adapted to
high speed work. Another reason mentioned is the fact,
that for making such a movement at such a high speed,
passive components, i.e., the non-contractile muscle elements, participate to a greater extent. Iga, George, Lees
and Reilly (2009). With the use of isokinetic dynamometry 15 year old soccer players compared favorably with
their contemporaries who were not regularly involved in
sport activities. For this purpose they were measured at
speeds of 60° × s–1 and 250° × s–1. While the difference in
PT at the speed of 60° × s–1 was only from 15 to 18%, at
the speed of 250° × s–1 soccer players were significantly
stronger by 25 to 37%. The results show that training
preparation and play performance itself in soccer require
the development of muscle strength, particularly at the
higher speed levels, which is not yet well developed in
adolescents from the common population.
We confirmed by observing the group of players in
our study that the age factor significantly influenced
the isokinetic strength level. A detailed comparison of
the PT with DL as well as NL between the categories
nearest to each other, with the help of the LSD post-hoc
test, confirmed statistically important differences in
PT at the speed of 60° × s–1 only between categories
U16 and U17 in the case of extension. Differences in
the case of flexion were “on the borderline of statistical
significance” (p = 0.062). The difference between the
categories U17 and U18 were statistically insignificant in
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Acta Univ. Palacki. Olomuc., Gymn. 2011, vol. 41, no. 2
flexion as well as extension. In the case of extension the
differences were on the borderline of statistical significance (p = 0.054). A more detailed analysis shows that
statistically significant differences in muscle strength between age categoriesʼ flexors and extensors at the speed
of 60° × s–1 and 180° × s–1 were influenced first and
foremost by a more pronounced trend of the strength
increase between the U16 and U17 categories. On the
other hand for the U17 and U18 categories at the speed
of 60° × s–1 for the DL extension we noted PT stagnation
and for the NL even a decrease (Fig. 1). This finding
suggests that within the oldest category monitored, sufficient attention is not given to strength training. With
regard to the importance of strength for accomplishing
specific movements in current soccer (Verheijen, 1998)
we consider this finding important. This detailed retrospective data analysis of the training process showed us
that the stagnation of strength in the U18 category was
not determined by the limit of muscle strength development, but by mistakes in strength training.
Inter-individual data analysis related to his or her
medical history furthermore helped us to determine
that the imbalance of the flexor and extensor knee joint
strength in the case of some of the observed players
can be the key factor for the development of repeated
micro traumas to the knee flexors. This misbalance in
the isokinetic dynamometry can be best expressed by
the examination of the flexor to the extensor strength
ratio – the so called H/Q ratio (Houweling et al., 2009;
Lehance et al., 2009; Sangnier & Tourney-Chollet,
2007). Differences in the strength between the U16 and
U18 groups are in accordance with the results of other
studies (Forbes et al., 2009). At the highest speed of
360°·s–1 a statistically significant difference was not confirmed in any case. During the result interpretation in
this speed it is also necessary to take into consideration
problems related to the validity and reliability of the isokinetic testing at high speeds (Dvir, 2004).
The growth of the PT connected to the young soccer
player’s chronological age growth is also mentioned in
the results of the Forbes, Bullers et al. (2009) study. In
the case of the PTʼs expression relative to the player’s
weight authors found out that the U18 group of players wasn’t significantly stronger than the U16 and U15
group of players. Kellis et al. (2001) measured isokinetic
strength at speeds of 60° × s–1, 120° × s–1 and 180° × s–1
with young Greek players and they determined a significant increase of muscle strength corresponding to
the player’s age from 10 to 18 years. When we compare average PT values during flexion, respectively, the
Fig. 1
Muscle strength in connection with age during extension at the speed of 60°·s-1
Average values of DL and NL extensions at speed 60°·s -1
(average and 95% confidence interval)
260
250
240
230
Nm
220
210
200
190
180
170
160
16
17
18
age
PT Ex Dom 60
PT Ex NDom 60
Legend: PT Ex Dom 60 – Peak Torque extension of the dominant leg at the speed of 60° × s–1, PT Ex Ndom 60 – Peak
Torque extension of the non-dominant leg at the speed of 60° × s–1
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Acta Univ. Palacki. Olomuc., Gymn. 2011, vol. 41, no. 2
extension for DL in the monitored player’s group in
the U17 category at the speed of 60° × s–1 (125 Nm,
resp. 226 Nm) with the values of the Greek players of
the same age (140 Nm, resp. 213 Nm), we can say that
our players lag behind the Greek players in hamstring
strength. It can be assumed that this condition can be
undesirable, particularly from the point of view of injury
prevention. Similarly Lehance et al. (2009) noted a statistically significant difference between Belgian players
in the U17 and U21 groups at the speed of 60° × s–1 for
the dominant DL. Average results in the U17 playersʼ
group were 128 Nm during flexion and 194 Nm during
extension, values not greatly different from the values
in the U17 group of our group of players. These authors
further state that in the expression of the isokinetic testing results of muscle strength compared to body weight,
17 year old highly trained soccer players have already
reached a muscle strength of the flexor and extensor of
the knee very close to their maximal level. From this
point of view there are the very interesting results of the
De Ste Croix et al. (2001) study. These authors came to
the conclusion that age is a non-significantly explanatory
variable of isokinetic knee torque once stature and mass
are accounted for. Gür et al. (1999) evaluated the difference between players up to 21 years of age and the adult
category of the Turkish soccer league at the speeds of
30° × s–1, 180° × s–1, 240° × s–1 and 300° × s–1 and except
for the lowest speed they found a significant difference
in the strength of the quadriceps and hamstrings only
for DL. In comparison, the muscle strength level for
Turkish soccer players up to 21 years old for DL was on
average 86 Nm for flexion and 144 Nm for extension.
The players in our U18 group reached an average value
of 85 Nm, respectively 119 Nm.
CONCLUSIONS
This study of the soccer players in the age group
from 16 to 18 years old evaluated the muscle strength
preparation by the young highly trained soccer players
in the Teen Sport Center SK Sigma Olomouc, CR. The
study confirmed that an increase in the speed of the
movement leads to a decrease in isokinetic strength. It
also confirmed that muscle strength increases with age.
In the group of 17 and 18 years old soccer players the
differences in the PT for flexors, resp. extensors of DL
and NL were not important. In the case of NL extensors
we recorded a small decrease in PT, which can indicate
a lack in the strength training for the oldest playersʼ
group. Results of the study partly deepen the knowledge
of the changes in muscle strength, with connection to
age, including its production at higher, more specific,
speeds. They showed the importance of diagnostics during systematic long term sport preparation, specifically
the importance of strength diagnostics and training in
contemporary soccer.
ACKNOWLEDGMENT
The study has been supported by the research grant
from the Ministry of Education, Youth and Sports of
the Czech Republic (No. MSM 6198959221) “Physical
Activity and Inactivity of the Inhabitants of the Czech
Republic in the Context of Behavioral Changes”.
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IZOKINETICKÁ SÍLA FLEXORŮ A EXTENZORŮ
KOLENA U ADOLESCENTNÍCH FOTBALISTŮ
A JEJÍ ZMĚNY S RYCHLOSTÍ POHYBU
A S VĚKEM
(Souhrn anglického textu)
VÝCHODISKA: Během dětství a dospívání dochází k nárůstu svalové síly. Dynamika změn svalové síly
a její příčiny nejsou dosud plně objasněny a mohou se
lišit mezi jednotlivci a specifickými skupinami. Znalost
aktuální úrovně izokinetické síly u sportovců různého
věku a disciplín představuje důležitou informaci z hlediska sportovního výkonu i zdravotní prevence.
CÍLE: Cílem studie je určit u skupiny elitních adolescentních fotbalistů (n = 45, věk = 17 ± 1,2 let, tělesná
výška 178,4 ± 5,3 cm, hmotnost 68,5 ± 7,6 kg) izokinetickou sílu flexorů a extenzorů kolena a posoudit, zda je
významně ovlivněna věkem a rychlostí pohybu.
METODIKA: Skupina hráčů byla dále rozdělena
podle věku do 3 podskupin – U16 (n = 16), U17 (n = 14),
U18 (n = 15). Unilaterální síla byla měřena izokinetickým
dynamometrem ISOMED 2000 (D. & R. Ferstl GmbH,
Hemau, Německo) v úhlové rychlosti 60° × s–1, 180° × s–1
a 360° × s–1. Hodnoceným parametrem byl maximální
moment síly (PT, Nm).
VÝSLEDKY: Výsledky ANOVA ukázaly, že v rámci
celé skupiny hráčů došlo u flexe i extenze k výraznému
poklesu PT se zvýšením rychlosti (dominantní končetina: p < 0,001 resp. P < 0,019, nedominantní končetina:
p < 0,001; resp. P < 0,001). Rozdíl v PT mezi věkovými
kategoriemi nebyl významný jak pro flexory, tak ani
pro extenzory při rychlosti 60° × s–1 (p = 0,005; resp.
p = 0,036) i při rychlosti 180° × s–1 (p = 0,036; resp.
p = 0,033). Významné rozdíly v PT mezi jednotlivými
kategoriemi byly potvrzeny jen v některých případech
a naopak byl zaznamenán nevýznamný pokles u extenzorů nedominantní končetiny.
ZÁVĚRY: Výsledky studie ukazují na stav silové
připravenosti elitních mladých fotbalistů a na možné
nedostatky v koncepci tréninku síly u předního klubu
České republiky. Výsledky přispěly k prohloubení znalostí o změnách síly s věkem a potvrdily důležitost uplatňování diagnostiky v rámci systematické dlouhodobé
sportovní přípravy.
Klíčová slova: fotbal, izokinetika, diagnostika, mládež.
Acta Univ. Palacki. Olomuc., Gymn. 2011, vol. 41, no. 2
Doc. PaedDr. Michal Lehnert, Dr.
Palacký University
Faculty of Physical Culture
tř. Míru 115
771 11 Olomouc
Czech Republic
Education and previous work experience
1980–1985 – Pedagogical Faculty, Palacký University:
Physical Education – Biology.
1985–1986 – Pedagogical Faculty, Palacký University:
PaedDr.
1987–1991 – teacher at elementary school.
1991–2007 – Faculty of Physical Culture, Palacký University: Assistant Professor.
1993–1997 – Faculty of Physical Culture, Palacký University: Ph.D.
Since 2008 – Faculty of Physical Culture, Palacký University: Associate Professor.
53
First-line publications
Lehnert, M., Novosad, J., Neuls, F., Langer, F., & Botek, M. (2010). Trénink kondice ve sportu. Olomouc:
Palacký University.
Lehnert, M., Stejskal, P., Háp, P., & Vavák, M. (2008). Load
intensity in volleyball game like drills. Acta Universitatis
Palackianae Olomucensis. Gymnica, 38(1), 51–58.
Lehnert, M., Janura, M., Jakubec, A., Stejskal, P., &
Stelzer, J. (2007). Reaction of the volleyball players
to the training microcycle with an increased strength
training volume. International Journal of Volleyball
Research, 9(1), 11–18.
Haník, Z., Lehnert, M. et al. (2004). Volleyball I (in
Czech). Prague: Czech Volleyball Federation.
Lehnert, M., Janura, M., & Stromšík, P. (2003). The
jump serve of the best servers on the Czech national
men’s volleyball team. International Journal of Volleyball Research, 6(1), 10–13.
Lehnert, M., Janura, M., Stromšík, P., & Vaverka, F.
(2001). 3D analysis of blocking in volleyball. In
J. Blackwell (Ed.), XIX International Symposium on
Biomechanics in Sports. San Francisco: University of
San Francisco.